1. Field
Embodiments described herein relate to electronically scanned array antennas and in particular to systems for controlling active electronically scanned array antennas.
2. Description of Related Art
An active electronically scanned array (AESA) antenna is an antenna composed of multiple radiating elements, or radiators, the relative amplitude and phase of which can be controlled, making it possible to steer the transmit or receive beams without moving the antenna. Such an antenna includes an array of radiators, or radiating elements. The AESA transmit and receive gain patterns may be uniquely set and may have different polarization states by applying different relative amplitudes and phases in the transmit and receive paths. Each radiator may be connected to a circulator for separating transmitted and received radio frequency (RF) paths having unique transmit/receive electronics. The electronics may include n-way combiner/dividers, for splitting the signal to be transmitted along the path to the radiators, and combining the received signals along the path from the radiators. The electronics may also include digitally controlled elements for adjusting the gain and phase of the signals propagating to or from the radiator, and for switching between the two signal directions, i.e., between the transmitting and receiving modes of the antenna. An electronically controlled attenuator, for example, may be adjusted to control the amplitude of the signal radiated by a radiator, or, if it is followed by a divider, the set of radiators fed directly or indirectly by that divider.
Conceptually, the digitally controlled components used to control amplitude, phase, and signal direction may be grouped into functional blocks referred to herein as digitally controlled RF signal transmission blocks. Supplying a digital control word to such a block through a digital RF block control bus may control the setting of every digitally controlled element in that block. An AESA antenna may contain several varieties of digitally controlled RF signal transmission blocks.
A dominant lobe of an antenna pattern may be referred to as a beam. Such a beam may have several characteristics: the beam direction, which may be characterized by azimuth and elevation angles, the beam width or spoiling, the frequency, and the polarization state. The set of characteristics defining the beam is known as the beam state. If an antenna is designed for both transmitting and receiving operation, then in addition to operating over a range of beam states, the antenna may, at any time, be either transmitting or receiving. The combination of the beam state an antenna is transmitting or receiving, as well as whether it is transmitting or receiving, will be referred to herein as the antenna state. The antenna state may be changed by sending a new digital control word to every digitally controlled RF signal transmission block in the AESA antenna.
The parameters for each digital control word may be recalculated each time the antenna state is to be changed. U.S. Pat. No. 5,008,680, for example, discloses a phase shift control circuit which uses control signals from a beam transform controller, as well as data stored in the phase shift control circuit, to determine the phase shift that the associated phase shifter will impart to the RF signal. In one embodiment, the phase shift control circuit contains multipliers and combiners to form products and sums of combinations of control signals and internally stored data. U.S. Pat. No. 4,445,119 discloses a phased array antenna subsystem in which a distributed beam steering microcomputer is collocated with each of a set of phase shifters. Each microcomputer is used to calculate the phase shift needed from the associated phase shifter to achieve a certain overall beam direction.
In systems requiring that calculations be performed each time the antenna state is to be changed, the rate at which antenna states can be changed may be limited by the time required to perform the calculations. If for example a multiplication is part of the calculation, and if a computer is used which requires some number of clock cycles to perform a multiplication, then the maximum rate at which the antenna state can be changed may be one at which that number of clock cycles elapses before each new antenna state change.
In another prior art embodiment, a small memory, capable of storing a small number of digital control words, e.g., eight digital control words, is associated with each digitally controlled RF signal transmission block. In operation, a central computer calculates all the parameters needed throughout the array for each state when the state is selected. These parameters are loaded into each small memory, during a programming phase, with digital control words corresponding, for example, to eight antenna states. After programming is complete, a beam steering controller may then rapidly switch to any of the eight available antenna states as commanded by the central computer by sending out the corresponding address, which causes the memory contents to be sent to the digitally controlled RF signal transmission block. In this embodiment, antenna state switching may be accomplished rapidly, as long as the switch is to one of the small number of programmed antenna states. A significantly longer delay is incurred when a new beam state must be calculated and then the memories reprogrammed to make one or more new antenna states available. This delay is unacceptable for some AESA applications.
In some modern AESA antenna applications, it is required to be able to switch the antenna state much more quickly to any antenna state supported by the antenna. Thus, there is a need for a system capable of switching among the full range of antenna states, and of doing so without incurring significant delays caused by the calculation time and time to load the required parameters for each antenna state.